Polygyny

Like polyandrous females, there are a variety
of ways in which males practice polygyny. There is a single
widely accepted theory as to why these different methods of
polygyny have evolved: Stephen Emlen and Lewis Oring have argued
that the extent to which a male can monopolize mating
opportunities depends on social and ecological factors that
affect the distribution of females (Emlen 354). Females that live together or aggregate at resource
patches can be monopolized economically by males, whereas
scattered females cannot, because as the size of a territory
grows, the cost of defending a site increases (Alcock 501).

Female Defense
Polygyny

The Emlen-Oring theory postulates that when
females aggregate, males will compete amongst each other for the
right to mate with this cluster. This theory is supported by
observations made of sheep, gorillas, lizards, and bats.

The benefits of female defense polygyny are
clear for both sexes. Males receive most of possible copulations
in a cluster they successfully protect, and females gain
protection for themselves and their offspring from predators (McCracken and
Bradbury 811-813). Females in these
clusters will often remain there well after the offspring are
born, providing care for dependent young.

In some species, males create clusters of
females by herding them together. This strategy is used by marine
amphipods that live in shallow bays. The male will leave his
"house" and search out single females. He will then
glue their houses to his own. The male will do this to 2-3
females, and then monopolize their sexual activity from that
point forward.

(A) The male marine amphipod alone, outside
of his house, and (B) in his house clustered with two other
females (Alcock
502).

Resource Defense
Polygyny

Females of many species do not live in tight
clusters, but a male may still be able to defend a territory that
makes him polygynous if the resources females need are clumped
spatially, permitting economical defense of a resource based
territory (Alcock
504). A safe location for eggs to
mature is a defendable resource for a host of animal species. For
example, the female African cichlid fish, Lamprologus
callipterus, will deposit eggs in an empty snail shell, then
follows them inside to guard them until they are ready to leave.
The male will scavenge for extra shells, add them to his midden
of shells, and defend his area. A male with a large, well
protected midden will enjoy great reproductive success.

Lamprologus callipterus,
the African cichlid fish, demonstrates that egg deposition
location is a defendable resource used frequently by polygynous
males to secure a cluster of females. (Alcock 505). View the Cichlid
Research Homepage.

The polygyny threshold model identifies
circumstances in which a female will gain more by mating with an
already mated male than with a bachelor. This model applies to
species in which the territory of a male contains useful
resources for the female and her offspring. A female is more apt
to pair with a mated male with great resources than a single male
with a poor territory. It appears the female's share of the
resources will be greater with the mated male despite having to
share him with many other females, thus increasing her
reproductive success (Orians 264).

Scramble
Competition Polygyny

In many species, females are widely dispersed
and it is very costly for males to attempt to guard individual
territories. When this scenario presents itself, males will
attempt to outrace each other to potential mates. Males that are
successful practice scramble competition polygyny. Thus mating
success in males practicing scramble competition polygyny is
reserved for those that are the most persistent, durable, and
perceptive, rather than the most aggressive and physically
imposing.

A very different type of scramble competition
polygyny occurs in species in which the female's breeding season
is highly compressed. The female horseshoe crab comes to shore to
lay eggs on certain nights when the tide conditions are
favorable. Males will gather off the nesting beaches in large
numbers and search out incoming females. When a male encounters a
single female, he will attach to her with special claws and
fertilize her eggs as she lays them onshore. It appears these
claws are highly specialized; they make the male difficult to
dislodge and put him in an optimal position to fertilize the
female's eggs.

A horseshoe crab moving up a beach and the
underside of a male horseshoe crab. The front claws (which appear
to be hanging down from the top of the shell) are larger and
slightly structurally different than the female's for use during
mating. Photos courtesy The
Assateague Naturalist.

The Lek

Sometimes males will defend territories too
small for there to be useful resources for a female to utilize.
As a result, males will cluster in a small area for display.
Females come to these display areas, leks, to mate. After
copulation females often depart the lek and never return. Often a
very small percentage of males will monopolize a large number of
sexual acts occurring in the lek. Why do subdominant males
aggregate in leks if there is a high probability that they will
not increase their chance at copulation? It is argued that males
aggregate in leks only after other mating systems have not paid
off as a result of wide and even dispersion of females. Why do
males choose to display so close to one another rather than in
their own isolated territories? There are three theories that
purport to explain this:

The Hotspot Hypothesis. Males aggregate at
certain points because females tend to travel along
certain routes that intersect at certain points
(hotspots).

The Hotshot Hypothesis. Males cluster
because subordinate males gather around highly attractive
males to increase their exposure to females, thus
increasing their likelihood of finding a mate.

The Female Preference Hypothesis.
Postulates that males cluster because females prefer
sites with large groups of males.

Lek size and copulation rates for the
lekking sandpiper. A) Males at larger leks attract more receptive
females than those at small leks, B) At leks of six or more
males, the number of attracted females levels off, lowering the
reproductive success of dominant males because of competition
from lower-ranking males. (Alcock 516).

The following description of a prairie chicken
lek is by Jim Metzner as part of the radio program "Pulse of the
Planet." The lek he describes is
approximately the size of a football field and contains about 20
males.

"And soon after that with a very first light, dozens of
males will gather and begin to all display in unison.
Periodically, fights will break out among neighboring males
at the edges of their territorial boundaries and then on
occasion, a female will begin to walk slowly through the lek.
Each male defends an area about ten, fifteen meters across
and as the female saunters across the lek each male begins
displaying furiously and accompanying the female through the
lek and up to the edges of the territories where she will
cross into yet another male's territory. At these boundaries,
often fights, punctuated by loud alarm calls, will frequently
break out among neighboring males. If the female will
frequently visit the lek a number of times before she chooses
a mate and after she chooses a mate, mating is very quick and
she leaves the lek and goes off and raises the young on her
own."